1. Field of the Invention
This invention relates to flexographic printing and more particularly to a flexographic printing plate having a plurality of ink carrying cells in the solids areas, and the method for making such plates.
2. Description of Related Art
Flexography is a direct rotary printing method that uses resilient relief image plates of rubber or other resilient materials including photopolymers to print an image on diverse types of materials that are typically difficult to image with traditional offset or gravure processes, such as cardboard, plastic films and virtually any type of substrate whether absorbent or non absorbent. As such it has found great applications and market potential in the packaging industry.
Flexographic printing plates are normally affixed onto a printing cylinder for printing. As shown in FIG. 1 an ink fountain pan 10 supplies ink to a metering roll 14. An optional doctor blade 12 may be used to wipe off excess ink from the metering roll to assist in controlling the amount of ink that is on the metering roll. The flexographic printing plates 16 are mounted on the printing cylinder 18. The material to be printed, usually supplied as a continuous web 19, is placed between the printing roll 18 and a backing roll 20. The flexographic printing plate is brought against the material typically with just sufficient pressure to allow contact between the relief image on the plate and the material printed.
Flexograhic printing plates can be made of either vulcanized rubber or a variety of radiation sensitive polymer resins, typically sensitive to ultraviolet radiation. A well known such flexographic photosensitive polymer resin plate is CYREL.RTM.. CYREL.RTM. is a registered trademark for a photopolymer printing plate, a product of E.I. DuPont de Nemours and Co. Inc. which was introduced in the mid seventies and has since found widespread acceptance by the printing industry.
Flexography printing is a printing process whereby ink is transferred through a metering roll to the relief portions of the printing plate and therefrom in a process akin to stamping from the relief plate areas to the printed surface. In order to produce good images it is essential that the ink applied to the printed surface is applied uniformly and predictably. This in turn requires that the relief areas in the flexographic plate carry ink in a uniform layer and in predictable amounts.
The prior art has attempted to solve this problem by using a special ink metering roll which is known as an anilox roll. Anilox rolls have on their surface a plurality of ink metering cells. These cells are small indentations arrayed in regular patterns of a predetermined frequency and of uniform depth and shape. Typically they are created by engraving the cylinder face by a mechanical process or by laser. The amount of ink delivered by the anilox roll is controlled by the screen size of the cells.
In operation ink is transferred from the ink well onto the anilox metering roll 14 filling the cells. The optional wipe blade 12 wipes off excess ink from the roll surface leaving only the cells filled. The ink from the cells is then transferred onto the flexographic plate relief areas as the anilox roll and the flexographic plate rotate in contact with one another.
Flexographic printing is what may be termed as a binary system. That is it either prints or it does not. Whenever relief areas contact the printed surface, one gets a substantially solid color area. To create a gray scale, a process called half-toning is used. This is a well known process wherein gray tones are reproduced by printing a plurality of minute solid dots per unit area and varying either the frequency of the dots per unit area or the size of the dots per unit area or both.
It has been observed, and is a well known problem in flexographic printing that solid areas, that is areas in the image where there are no half tone dots, appear to print with less saturation and somewhat less uniformity than halftone areas representing dark image areas. Thus an area with a dot coverage of 95% to 98% appears darker than a solid area (100%). Furthermore, solid flexographic image areas tend to show a "halo" around the solid area, that is, a darker border around the solid image area.
As mentioned earlier, flexography's primary application is packaging. Due to product competition, the market requirements on the printing quality of the images on the packaging are becoming very stringent. There is thus a need for flexographic printing plates that alleviate these problems and deliver a better quality image.